Mounting evidence points to mobile devices replacing social networks as the dominant platform for casual games -- a trend underlined by audiences for the top Facebook titles shrinking rapidly.

In August, most of the top titles from major social game publishers like Zynga, Electronic Arts, and Disney/Playdom saw their daily active user numbers suffer double-digit declines month-to-month, according to Cowen and Company analyst Doug Cruetz.

Meanwhile, free-to-play titles on smartphones and tablets had their most dominant month yet -- across the three top 20 grossing games lists for iPhone, Android, and iPad last month, 56 out of the 60 titles use the free-to-play model.

"We believe that over the last several months, trends in the casual digital gaming space have swung decisively towards mobile and away from PC-based social gaming, at least in Western markets," says Cowen.

A number of Facebook's biggest developers have been shifting their focus, if not completely migrating, to mobile in the last year. Top Girl maker CrowdStar, for example, decided to halt all development on social titles, and work on mobile games instead.

And Zynga, the largest developer on Facebook in terms of audience size, for example, has put plenty of effort into growing its mobile presence recently. It even spent a reported $210 million last March to acquire Omgpop, the studio behind one of the biggest mobile game properties at the time, Draw Something.

Just yesterday, Facebook's CEO Mark Zuckerberg admitted that users are more engaged and spending more time on mobile than on desktops, and that the company believes it can make a lot more money by targeting people accessing the social network on their smartphones.

"A lot of the development and the energy in the eco-system is not going towards building desktop stuff anymore, it's going towards building mobile stuff," says Zuckerberg. "We're able to integrate and we're helping a lot of folks build great mobile experiences - that's the future."